fix(mcp) : return CallToolResult to prevent structuredContent serialization issue

Tools now return CallToolResult directly instead of Content arrays,
preventing the MCP SDK from auto-generating structuredContent as a
JSON array (which Claude Code rejects — expects a JSON object/record).
Also adds Accept header to test helpers and SSE response parsing.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/Mcp/Tool/CustomField/ListCustomFieldValuesTool.php

@@ -8,6 +8,7 @@ use App\Entity\CustomFieldValue;
 use App\Mcp\Tool\McpToolHelper;
 use Doctrine\ORM\EntityManagerInterface;
 use Mcp\Capability\Attribute\McpTool;
+use Mcp\Schema\Result\CallToolResult;
 
 #[McpTool(
     name: 'list_custom_field_values',
@@ -23,7 +24,7 @@ class ListCustomFieldValuesTool
         private readonly EntityManagerInterface $em,
     ) {}
 
-    public function __invoke(string $entityType, string $entityId): array
+    public function __invoke(string $entityType, string $entityId): CallToolResult
     {
         $entityType = strtolower($entityType);